A divided Republican Party erupted into open and bitter warfare on Thursday as its two previous presidential nominees delivered an extraordinary rebuke of its current front-runner, Donald J. Trump, warning that his election could put the United States and its democratic system in peril. In a detailed, thorough and lacerating assault on Mr. Trump and the angry movement he has inspired, Mitt Romney, the party’s nominee in 2012, attacked him as “a fraud” and “a phony” who would drive the country to the point of collapse.